This invention relates to novel platic compositions having enhanced environmental degradability.
The advent of plastics has given rise to improved methods of packaging goods. For example, polyethylene and polypropylene plastic films, bags, bottles, styrofoam cups and blister packages have the advantages of being chemically resistant, relatively unbreakable, light in weight and translucent or transparent. The increasing use of plastics in packaging applications has created a serious waste disposal problem. Burning of these plastic materials is unsatisfactory since it adds to air pollution problems.
Unlike some other packaging materials, such as paper and cardboard, plastics are not readily destroyed by the elements of nature. Thus, burying them is not an effective means of disposal, and can be expensive.
Plastics are biologically recent developments, and hence are not easily degradable by micro-organisms which attack most other forms of organic matter and return them to the biological life cycle. It has been estimated that it may take millions of years for organisms to evolve which are capable of performing this function. In the meantime, plastic containers and packaging films are beginning to litter the countryside after being discarded by careless individuals.
One means suggested for combating the plastic waste disposal problem has been the use of plastic compositions which degrade upon exposure to ultraviolet light. For example, reference is made to British patent specification No. 1,128,793, which describes ethylene-carbon monoxide copolymers which undergo rapid deterioration when subjected to natural sunlight or artificial sources of ultraviolet radiation.
The suggested use of ultraviolet-degradable polymeric compositions for packaging applications is attractive, in the sense that materials exposed to sunlight, such as roadside litter, will undergo accelerated environmental deterioration. However, the deterioration of such materials will not necessarily be rapid if they are not exposed to sunlight, as, for example, in the case of waste materials which are buried or covered by other materials in the course of waste disposal.
An alternative approach involves the use of oxygen, rather than sunlight, as the primary initiator of polymer degradation.
The enhancement of the rate of environmental deterioration of plastics through the use of oxidation-promoting additives is known in the prior art. For example, the preparation of degradable polyolefin films containing certain organic derivatives of transition metals is described in U.S. Pat. No. 3,454,510.
In the course of experimental investigations of the selective degradation of polyolefins, it was unexpectedly discovered that certain organic derivatives of transition metals display exceptional reactivity in promoting the non-photochemical degradation of polyolefins. An example is cobalt tallate, which is far more reactive than cobalt stearate in promoting the oxidative deterioration of polypropylene or polyethylene. Further experimentation established that transition-metal derivatives of highly unsaturated fatty acids (or fatty-acid mixtures) are generally much more effective pro-oxidants relative to the corresponding salts derived from saturated fatty acids, such as the stearates, or relatively less unsaturated fatty acids.